Regeneration method for process which removes hydrogen sulfide from gas streams

ABSTRACT

A process is provided for the removal of hydrogen sulfide out of a gaseous stream ( 22 ), such as a natural gas, by contacting the hydrogen sulfide containing gas with a sorbing liquid ( 26 ) containing a tertiary amine so that the hydrogen sulfide is sorbed into the liquid in absorber ( 11 ) and transferring the sorbing liquid/hydrogen sulfide mixture to a reactor ( 15 ) where the tertiary amine promotes the conversion of the hydrogen sulfide into polysulfide via reaction with sulfur; transferring the polysulfide solution from the reactor ( 15 ) to a regenerator ( 10 ) where polysulfide is converted into elemental sulfur via reaction with air ( 9 ); transferring at least a portion of the solution ( 25 ) containing elemental sulfur, as well as sulfate and thiosulfate species, into a mixture ( 36 ) where it is contacted with gaseous ammonia which reacts with the sulfate and thiosulfate species to produce ammonium sulfate and ammonium thiosulfate which are removed from the solution while the remaining portion of solution ( 25 ) is transferred to a sulfur recovery unit ( 14 ). That portion of the solution which has been subjected to ammonium sulfate and ammonium thiosulfate removal is rejoined with that portion of the solution ( 25 ) being forwarded to sulfur recovery unit ( 14 ). The solution from the sulfur recovery unit ( 14 ) is recycled back to the absorber ( 11 ).

This application was filed under 35 U.S.C. 371 as PCT/US99/16500 on Jul.21, 1999, which claims 35 U.S.C. 119(e) benefit of U.S. provisionalapplication No. 60/093,598 which was filed on Jul. 21,1998.

FIELD OF INVENTION

This invention relates generally to processes and systems for removinghydrogen sulfide from a gaseous stream. More specifically the inventionrelates to improvements in a known process and system wherein hydrogensulfide is removed from a gaseous stream, using as an oxidizing agent anonaqueous scrubbing liquor in which are dissolved sulfur and areaction-promoting base.

DESCRIPTION OF PRIOR ART

In the present inventor's U.S. Pat. Nos. 5,733,516 and 5,738,834 (theentire disclosures of which are hereby incorporated by reference), aprocess and system are disclosed which use a sulfur-amine nonaqueoussorbent (SANS) for removal of hydrogen sulfide (H₂S) from gas streams.Pursuant to the said process, a sour gas stream containing H₂S iscontacted with a nonaqueous sorbing liquor which comprises an organicsolvent for elemental sulfur, dissolved elemental sulfur, an organicbase which drives the reaction converting H₂S sorbed by the liquor to anonvolatile polysulfide which is soluble in the sorbing liquor, and anorganic solubilizing agent which prevents the formation of polysulfideoil-which can tend to separate into a separate viscous liquid layer ifallowed to form. The sorbing liquor is preferably water insoluble asthis offers advantages where water soluble salts are desired to beremoved. Hydrogen sulfide (H₂S) gas is sorbed into this sorbing liquorwhere it reacts with the dissolved sulfur in the presence of the base toform polysulfide molecules. This reaction decreases the equilibriumvapor pressure of H₂S over the solution, thus providing more efficientscrubbing than a physical solvent. The liquor is then sent to a reactorwhere sufficient residence time is provided to allow the polysulfideforming reactions to reach the desired degree of completion—i.e.,resulting in a nonvolatile polysulfide which is soluble in the sorbingliquor. From the reactor, the liquor flows to a regenerator where thesolution is oxidized (e.g., by contact with air), forming dissolvedelemental sulfur and water (which, being insoluble, is rejected from thesolution either as an insoluble liquid layer or as water vapor exitingthe overhead of the regenerator or absorber). The temperature of theliquor, which up to this point is sufficient to maintain the sulfur insolution, is then lowered, forming sulfur crystals, which are easilyremoved by gravity settling, filtration, centrifuge, or other standardremoval method. Enough sulfur remains dissolved in the liquor followingseparation of the sulfur crystals that when this solution is reheatedand returned to the absorber for recycling in the process, a sufficientamount of sulfur is present to react with the inlet H₂S gas.

The process and system for removal of hydrogen sulfide from a gaseousstream in accordance with my U.S. Pat. Nos. 5,733,516 and 5,738,834patents thus utilize a nonaqueous sorbent liquor comprising a solventhaving a high solubility for elemental sulfur, and a sufficienttemperature so that solid sulfur formation does not occur either in thehydrogen sulfide absorber or in the air-sparged regenerator of thesystem utilized for carrying out the process. The solvent generally canhave a solubility for sulfur in the range of from about 0.05 to 2.5, andin some instances as high as 3.0 g-moles of sulfur per liter ofsolution. The temperature of the nonaqueous solvent material ispreferably in the range of about 15.degree. C. to 70.degree. C. Sulfurformation is obtained, when desired, by cooling the liquor proceedingfrom the air-sparged regenerator. This can for example be effected at asulfur recovery station by cooling means present at the station. Thesolvent is thereby cooled to a sufficiently low temperature tocrystallize enough solid sulfur to balance the amount of hydrogensulfide absorbed in the absorber. The solubility of elemental sulfurincreases with increasing temperature in many organic solvents. The rateof change of solubility with temperature is similar for many solvents,but the absolute solubility of sulfur varies greatly from solvent tosolvent. The temperature change necessary to operate the process willvary primarily with the composition of the sorbent the flow rate ofsorbent, and the operating characteristics of the recovery station. Formost applications, a temperature difference of 5.degree. C. to20.degree. C. is appropriate as between the temperature of the solventmaterial at the absorber/reactor and temperature to which the saidsolvent is cooled at the sulfur recovery station; but the temperaturedifference can in some instances be as little as 3 .degree. C. or asmuch as 50 .degree. C. The nonaqueous solvent comprises a solventselected from the group consisting of 1, 2, 3, 4 tetrahydronaphthalene,N, N dimethylaniline, diphenyl ether, dibenzyl ether, terphenyls,diphenylethanes, alkylated polycyclic aromatics, and mixtures thereof.

In order to obtain a measurable conversion of sulfur and hydrogensulfide to polysulfides, the base added to the solvent must besufficiently strong and have sufficient concentration to drive thereaction of sulfur and hydrogen sulfide to form polysulfides. Mostprimary, secondary and tertiary amines are suitable bases. Moreparticularly, amines which comprise nitrogen connected to alkane groups,alkanol groups, benzyl groups, or hydrogen (but not to phenyl) aresuitable. It should be noted that while the solvent utilized requiresthe addition of a base to promote the reaction of sulfur and hydrogensulfide to form polysulfides, the base and the solvent may be the samecompound.

The base may be a tertiary amine. Polysulfide compounds formed in thepresence of tertiary amines are much more easily converted to sulfur byair during the regeneration step than those formed from primary aminesor secondary amines. The base is more preferably selected from the groupconsisting of 2-(dibutylamino) ethanol, N-methyldicyclohexylamine,N-methyl-diethanolamine, tributylamine, dodecyldimethylamine,tetradecyldimethylamine, hexa-decyldimethylamine, diphenylguanidine,alkylaryl polyether alcohols, and mixtures thereof. The base is presentat concentrations of about 0.01M to 2.0M. Of the bases cited,2-(dibutylamino) ethanol and N-methyldicyclohexylamine are mostpreferred, and are preferably present at concentrations of about 0.5 to1.0 M.

The nonaqueous sorbing liquor, in addition to including a solvent havinga high solubility for sulfur, and a base, comprises an agent suitablefor maintaining the solubility of polysulfide intermediates which mayotherwise separate when they are formed during operation of the process.Such solubilizing agent is preferably selected from the group consistingof benzyl alcohol, benzhydrol, 3-phenyl-1-propanol, tri(ethyleneglycol), and mixtures thereof.

During operation of this SANS process, most of the H₂S removed from thegas stream is converted to elemental sulfur. However, a small fractionof the absorbed H₂S is oxidized further to thiosulfate and sulfatespecies, which are soluble in the sorbent (probably as the salt of theprotonated amine). Unrestricted buildup of the sulfur oxyanion speciescan eventually cause an increase in the viscosity of the solution andreduce absorption rates. In its protonated form, the amine is not aneffective catalyst for the absorption of H₂S and subsequent conversionto polysulfidc. Therefore, the regeneration process must liberate thefree amine from its protonated form as well as remove the sulfate andthiosulfate. Byproduct salt buildup can also result in uncontrolleddeposition of sulfate or thiosulfate salts in the system. Therefore,some means of efficient removal of thiosulfate and sulfate from thecirculating solution must be employed.

Washing a portion of the sorbent with water or a mildly alkalinesolution is disclosed in the U.S. Pat. Nos. 5,733,516 and 5,738,834patents, and is a workable method. In this case, the sulfate andthiosulfate are extracted into an aqueous phase where they are moresoluble. Making the aqueous phase alkaline retards the transfer of aminefrom the nonaqueous phase if the “free base” form of the amine is waterinsoluble. However, such a washing step has several disadvantages. Inpractice, it is difficult to avoid emulsion formation and transport ofliquid water throughout the system. In addition, the need for amines andother solvent components to be water-insoluble is an undesirablerestriction on selection of these components. Furthermore, large amountsof water may have a deleterious effect on the SANS process operation.

SUMMARY OF INVENTION

Now in accordance with the present invention, it has been found that theforegoing problems of the prior art SANS process can be overcome by useof a new method for removing thiosulfate and sulfate from the SANSprocess solution. This method is based on addition of gaseous ammonia tothe process solution. Surprisingly, it has been found that the ammoniumsalts of thiosulfate and sulfate are quite insoluble in the nonaqueousSANS solution, in contrast to their high solubility in aqueoussolutions. Therefore, bubbling ammonia into a SANS solution containingthese salts results in nearly instantaneous formation of solid ammoniumsulfate and ammonium thiosulfate which precipitate from the solution,thereby allowing their removal by settling, filtration, or other commonsolid/liquid separation methods. The reaction (for sulfate) can bewritten as follows, with B representing the amine (HB³⁰ is then theprotonated amine):

(HB)₂SO₄+2NH₃→(NH₄)₂SO₄↓+2B

Since no water is used, this method avoids the problem of emulsionformation and transport of water around the system. In addition, thereactions appear to be essentially quantitative, and since ammonia isquite inexpensive, the removal process is economically favorable. Watersoluble components can be used in the sorbent without losing them to thewater wash or using other separation steps to recover them from the washwater of the earlier method.

Removal of the sulfate and thiosulfate with ammonia causes a decrease inthe electrical conductivity of the solution. Thus measurement of thesolution conductivity can provide a means of monitoring the status ofbyproduct removal and thus a means of controlling the rate of additionof ammonia.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings appended hereto:

FIG. 1 is a is a schematic block diagram of a system operating inaccordance with the present invention;

FIG. 2 is a schematic block diagram of a test system operating inaccordance with the present invention; and

FIG. 3 is a graph depicting the variation of byproduct concentrationswith time, for the test system of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a schematic block diagram appears of a system 20 which may beused in practice of the present invention. The system 20 is generallysimilar to the systems illustrated in my aforementioned U.S. Pat. Nos.5,733,516 and 5,738,834 patents, with the important exceptions of theportions of the system which enable removal of the undesired sulfate andthiosulfate salts. In a typical application of the invention, a gaseousstream 22 to be treated by the process of the invention is a natural orother fuel gas which typically includes 0.1 volume % to 5.0 volume % ofhydrogen sulfide, which component for environmental and other reasons isdesired to be minimized in or substantially removed from the gas stream.A more common parlance in the art is to measure the degree ofcontamination of a gas stream sought to be treated in terms of its dailyproduction of sulfur. When viewed in this way, the streams to be treatedby the invention will generally be those that produce 0.1 to 30 tons/dayof sulfur. In a representative case where input stream 22 comprises anatural gas, it is provided to system 20 at a pressure of around 1,000p.s.i. The stream 22 is passed into and through an absorber 11 where thehydrogen sulfide is effectively removed so that the output stream 24 issubstantially free of hydrogen sulfide—typically concentrations ofhydrogen sulfide in output stream 24 will be less than 4 ppm by volume.

Absorber 11 is a conventional liquid-gas contact apparatus at which theinput gas stream 22 to be purified is passed in counter-current or otherrelation to a liquid sorbent liquor 26. Absorber 11 may for example takethe form of a tower which is packed with porous bodies so as to providea high surface area for the gas-liquid contact. Other absorber apparatusas are known in the art can similarly be utilized. Pursuant to theinvention, the sorbent liquor 26 comprises a preferably nonaqueoussolvent having a high solubility for sulfur, typically in the range offrom about 0.05 to 2.5 g-moles of sulfur per liter of solution. Sorbentliquor 26 as provided to absorber 11 includes sulfur dissolved in thenonaqueous solvent in the range of from about 0.05 to 2.5 g-moles ofsulfur per liter of solution, together with a base (such as theaforementioned tertiary amines) having sufficient strength andsufficient concentration in respect to that of the hydrogen sulfide andsulfur to drive a reaction between the sulfur and hydrogen sulfide whichresults in formation of one or more nonvolatile polysulfides which aresoluble in the solvent. In order to provide sufficient residence timefor the reactions forming the polysulfidc, a reactor vessel 15 ispreferably provided downstream of the absorber. This vessel can also bephysically present in a delay section at the base of the absorber tower.The reactor vessel can be of conventional construction such as a plugflow reactor. Total residence time for the reaction, whether carried outin the absorber alone, in the absorber and the reactor, or in thereactor alone, can be in the range of 5 to 30 minutes, with 15 minutesor so being typical. The polysulfide remains in solution in the solvent,and the spent sorbing liquor including the dissolved polysulfide isconveyed via line 13 to a regenerator 10. Since it is possible forcertain polysulfide intermediates to separate as their concentrationincreases during practice of the invention (e.g., an amine-polysulfide“red oil” where the aforementioned base is a tertiary amine), apolysulfide solubilizing agent is preferably also present in sorbingliquor 26. Benzyl alcohol is a typical such solubilizing agent; howeverother agents such as benzhydrol, glycol, and mixtures of these severalagents are suitable; and in addition the solubilizing function can beaccomplished in some instances by one of the other components of thesorbent, such as the nonaqueous solvent or the base.

It is to be appreciated that the spent sorbing liquor provided toregenerator 10 is entirely provided as a liquid phase. Substantially nosolid sulfur particles are present as could cause blockages or otherdifficulties either at the absorber or in other portions of the systemproceeding regenerator 10. At regenerator 10, the sorbing liquor at atemperature in the range of 15.degree. C. to 70.degree. C. is oxidizedby contacting with an oxygenating gas, as for example by contacting witha counter current stream of air, or other means. Typically, for example,the sorbing liquor can be contacted with an ascending upwardly spargedair stream from supply line 9, which air is at a temperature of15.degree C. to 70.degree. C. Residence time in the regenerator istypically on the order of 15 to 45 minutes, and results (in the presenceof the aforementioned base) in the dissolved polysulfide being oxidizedinto elemental sulfur. More than 85% conversion of the polysulfide toelemental sulfur is achieved with the surprisingly short residence timeindicated. Because of the high solubilizing characteristics of thesolvent, and of the temperature of the solvent at regenerator 10,substantially no precipitation of the sulfur occurs at the regenerator,thereby continuing to avoid clogging and similar problems as often occurwhere slurries are developed. The sorbing liquor is thereupon dischargedfrom the regenerator and proceeds through a line 25 toward a sulfurrecovery station 14. Air and water vapor are discharged from regeneratorat vent 27. This vent stream will likely be of acceptable environmentalquality, but can be catalytically combusted if it contains large amountsof benzene or other volatile organic compound contaminant sorbed fromthe inlet gas.

Downstream of regenerator 10 and upstream of sulfur recovery station 14,a slipstream 30 diverted through pump 37. The sorbent liquor proceedingfrom regenerator 10 via line 25 contains undesired thiosulfate andsulfate species, which are soluble in the sorbent (probably as the saltof the protonated amine). In order to remove these undesired speciesammonia gas is added to the liquor, preferably by bubbling the gasthrough the liquid. This can be accomplished e.g. by means of a simpleinline mixer 36. This results in nearly instantaneous formation of solidammonium sulfate and ammonium thiosulfate which precipitate from thesolution. The reaction (for sulfate) can be written as follows, with Brepresenting the amine (HB⁺ is then the protonated amine):

(HB)₂S0 ₄+2NH₃→(NH₄)₂S0 ₄↓+2B

Temperatures for the reaction at mixer 36 can typically be in the rangeof from about 10 to 80° C., with the range of 15 to 70° C. being moretypical. The liquor and precipitates are passed to a solid/liquidseparator 38 thereby allowing removal of the solids by settling,filtration, or other common solid/liquid separation methods, with theliquid phase, i.e., the nonaqueous liquor, i.e., the now regeneratedsorbent being passed via line L19 to sulfur recovery system 14, andultimately being recycled to the absorber 11 for reuse in the cycle.

At or just prior to recovery station 14, the sorbing liquor is cooled toa sufficiently low temperature to enable solid sulfur to beprecipitated. The sorbing liquor discharged from regenerator willtypically have a temperature between 15.degree. to 70.degrees C. Thistemperature may be reduced as the sorbing liquor proceeds through line25 but does not reach a temperature at which sulfur precipitation occursuntil it approaches or reaches station 14. In any event, station 14 maycomprise a cooling means such as by refrigeration or heat exchange, withthe objective of reducing the temperature of the sorbent to that neededto precipitate enough sulfur to balance for the sulfur being added tothe sorbent by the hydrogen sulfide. The precipitated sulfur, as it isformed from a nonaqueous solvent, generally has a larger crystal sizeand a higher purity and better handling characteristics than suchproperties for sulfur precipitated from aqueous solution. Theprecipitated sulfur is separated from the sorbent by separating meanswhich form part of recovery station 14 or which can be immediatelydownstream of station 14. Separation can be accomplished by filtration,and/or settling and/or by centrifugation. The recovered sulfur atstation 14 can be purified at a sulfur purification station 18. Residualtraces of organic solvent on the sulfur crystals are removed with asolvent wash loop. Methanol can be used for such purpose, and can berecovered, distilled off and recycled in the loop.

Pumps 12 and 17 are shown positioned in the system 20 to enablecirculation of the sorbent in the manner shown—these and/or other pumps(such as pump 37) can be otherwise situated within the system to sustainthe desired circulation. A heating station 16 can be provided betweensulfur recovery station 14 and absorber 11 to bring the sorbent back toa temperature appropriate for dissolution of the sulfur that remainswith the sorbent as it is returned to absorber 11. Supplemental heatingmeans can also be provided at other points in the system to assure thatthe temperature remains above the sulfur precipitation point, i.e.,until the sorbing liquor reaches the point in its circulation where suchprecipitation is desired.

The quantities of ammonia which are added to the liquor at 36 are suchas to reduce the concentration of the sulfate and thiosulfate speciesbelow a predetermined level. In general it is desirable to reduce theconcentration to 0.05M or less. Since the said species act to tie up theamine and make it unavailable for its primary purposes in the invention,the more direct objective is to assure that the free amine concentrationis above a predetermined level. In practice, one can treat with ammoniain such quantities as to reach and maintain stable “steady state”concentrations of sulfate and thiosulfate, and then maintain asufficient flow rate of ammonia to maintain the sulfate and thiosulfateconcentrations at or below their set points. Similarly one canperiodically measure the concentration of free amine in the liquor andcontinue to precipitate and remove the undesired sulfate and thiosulfatespecies until the free amine rises to a level such that the steady stateflow rate of free amine in the liquor will be in a desired ratio to theH₂S entering the absorber. Each gmole of sulfate (or thiosulfate) isassociated with two gmoles of protonated amine, making this amineunavailable for reaction. At least two gmoles of amine per gmole ofincoming H₂S are needed to promote the desired reactions. The requiredconcentrations depend on the circulation rate of the liquor as well asother considerations. But basically, at steady state, enough ammonia isadded that the molar flow rate of free amine entering the absorber 11 isat least twice the molar flow rate of H₂S entering the absorber. Ahigher ratio may actually be used to speed up the desired reactions.

Typically at steady state operation the flow rate of free amine enteringthe absorber (on a mote basis) will be in a ratio to the entering (molarflow rate of) H₂S of the order of 2:1 to 8:1, although somewhat higheror lower ratios can be appropriate, i.e. the ratio could be less than2:1 if 100% removal is not necessary, or more than 8:1 where smallerabsorbers are used or for very high removals.

The absolute amount of ammonia needed will depend on the “make rate” ofsulfate+thiosulfate, which is some fraction of the incoming H₂S.Typically, the “make rate” of sulfate+thiosulfate for the process of the516 patent is about 0.05 gmole salt per gmole H₂S converted to S. Insuch a case one would require 2×0.05 gmole of NH₃ per gmole H2Sabsorbed. However the process of the present invention generallyexhibits an order of magnitude lower make rate.

While the ammonia in FIG. 1 is shown as added upstream of the sulfurrecovery station 14, and while this is an advantageous point for suchaddition, the ammonia can be effectively added at other pointsdownstream of the contacting of the liquor with an oxidizing gas. Thusfor example the ammonia addition could be effected immediatelydownstream of the sulfur recovery station 14. One of the advantages ofadding the ammonia upstream of the sulfur recovery, is that in suchevent the solids consisting of both the sulfur and the sulfate andthiosulfate salts, can be collected together. In some uses such as inagricultural supplements, such a combination of components is deemeddesirable. Similarly it will be appreciated that the two solid streamsproceeding from solid/liquid separator 38 and from sulfer recoverystation 14 (or from the purification means 18), could be combined into asingle solids stream.

EXAMPLES

Batch Test with Synthetic Byproduct Solution. To simulate a byproductsolution containing sulfate, a 0.25 mL aliquot of concentrated sulfuricacid was added to 50 mL of a solution of 7% (v/v)N-methyldicyclohexylamine, 25% benzyl alcohol and 68% Therminol 59(Monsanto trade mark). Before the sulfuric acid was added, the solutionpH was 8.2 and the conductivity was 0.007 microS (microS=microSiemens);after the addition, the pH was 6.18 and the conductivity was 0.53microS. The mixture in this reaction vessel was seeded with 1.00 g ofammonium sulfate solids. Ammonia gas at a flow rate of 20 cc/min wasthen added to the solution while monitoring the pH via a glass electrodeand the conductivity of the solution using a conductivity probe. Duringthis addition, the solution conductivity fell and the pH increased. Thefinal conductivity was 0.05 microS and the pH rose to 9.28 after theaddition of 180 cc of ammonia gas over a nine-minute period. Thedecrease in conductivity is indicative of the removal of the protonatedamine sulfate and thiosulfate salts. The increase in pH corresponds toliberation of the free amine from its protonated form.

Gelatinous white solids precipitated from solution during the ammoniaaddition. Additional ammonium byproduct solids accumulated onto the seedcrystals rather than the vessel walls and the pH and conductivity probes(as happens without seed crystals). After the ammonium solids wererecovered from the reaction vessel, rinsed, dried, and weighed, 1.9627 gof solids were measured. This represents a yield of 81% of thetheoretical increase in mass due to precipitated ammonium sulfate whenthe initial 1.00 g of ammonium sulfate seed crystal is taken intoaccount.

Batch Regeneration of Spent Bench-Scale Run Solution. The reactionvessel and experimental setup was the same as that described aboveexcept that actual spent sorbent solution from a system of the typedisclosed in my U.S. Pat. No. 5,738,834 was used (following 118 hours ofexposure to H₂S and regeneration with air). The solution was not seededwith ammonium sulfate solids prior to adding ammonia gas to thesolution. Ammonia gas at about 20 cc/min was added to the solution whilemonitoring the pH and the conductivity. After addition of ammonia for 23minutes, the conductivity fell from 13.0 microS to 0.49 microS and thepH rose from 3.71 to 9.66. Gelatinous white solids precipitated fromsolution, some accumulated on the pH probe and attached to the glasswalls of the vessel (illustrating the desirability of adding seedcrystals).

This batch run was repeated except that the starting spent reactionsolution was seeded with 1.00 gram of ammonium sulfate solids prior toadding ammonia gas. Ammonia gas at 20 cc/min was added to the solutionwhile monitoring the pH and the conductivity. After ammonia was addedfor 22 minutes, the conductivity fell from 12.5 microS to 0.22 microS,and the pH rose from 4.22 to 9.36. The experiment was initiated at roomtemperature. During this experiment, the temperature of the testsolution rose from 23° C. to 30.5° C. Apparently heat is liberatedduring ammonium byproduct formation. Gelatinous white solidsprecipitated from solution, however, none accumulated on the pH probe oron the glass walls of the vessel.

Continuous Flow Bench-Scale Run with Controlled NH₃ Addition. A run wasconducted to demonstrate that the process can be continually regeneratedby ammonia addition. Byproducts were removed by ammonia gas additionwhich was automatically controlled by monitoring the solutionconductivity. A schematic diagram of the process is shown in FIG. 1.

The sorbent contained 0.5 M sulfur, 0.4 M piperdineethanol, and 36%phenethyl alcohol dissolved in Therminol 59 (Monsanto trade name). Thegas stream was composed of 57% CO₂ and 43% H₂S. The process controlcomputer program and the process hardware were modified to automaticallycontrol the addition of ammonia to the process and thus continuallyregenerate the solution. The computer program controlled ammoniaaddition based on the conductivity of the sorbent. When the conductivityin the settler exceeded 5.0 microS, the ammonia valve was opened,allowing ammonia gas to flow into the process stream. When theconductivity fell below 4.0 microS, the ammonia gas valve was closed andammonia addition was stopped.

The variation of byproduct concentrations with time are shown in FIG. 2.The initial concentrations of sulfate and thiosulfate are high becausewe first operated the unit for 33.3 hours without adding ammonia inorder to build up the byproduct level. Once ammonia addition wasinitiated, the sulfate and thiosulfate concentrations decreased steadilyfor the first 140 hours because we operated the ammonia additioncontrols at maximum conductivity set point of 2.0 microS. This resultedin the addition of more ammonia than was needed to compensate for thebyproduct formation rate. Starting at 140 hours, the ammonia additionsystem was operated at a maximum conductivity set point of 5.0 microS,resulting in relatively steady concentrations of sulfate andthiosulfate.

Potentiometric titrations were performed on samples gathered from thebench-scale process before and after the addition of ammonia.Potentiometric titration plots of pH versus volume of standard acidadded clearly show that the amount of free amine is substantiallyreduced by the buildup of sulfate and thiosulfate, corresponding to 2moles of amine per mole of sulfate or thiosulfate. Potentiometrictitrations done after the addition of ammonia show that the free aminecontent has been essentially restored.

Inline filters were used to capture 948 grams of ammonium byproductsolids during this run. Assuming that the solid is exclusively ammoniumsulfate, this mass accounts for 89% of the ammonia added to the process.

Verification of Byproduct Solids from Ammonia Addition. Solids formed inthe process solution were sampled and analyzed by x-ray diffraction.Samples were taken from spent sorbent solutions for processes run inaccordance with U.S. Pat. No. 5,733,516, and the batch-scale testsduring which sulfuric acid was added to the process stream to simulatebyproduct production. All of the results indicated that the solidmaterial formed on ammonia addition is composed of ammonium sulfate andto a lesser extent ammonium thiosulfate. Some sulfur was also noted, aswould be expected from residual crystallization in the cooler regions ofthe process. Ion chromatographic analyses on extracts of the solids alsoconfirmed the presence of sulfate and thiosulfate.

While the invention has been set forth in terms of specific embodimentsthereof, it will be appreciated in view of the instant disclosure thatnumerous variations upon the invention are now enabled to those skilledin the art, which variations yet reside within the present teachings.Accordingly the present invention is to be broadly construed, andlimited only by the scope and spirit of the claims now appended hereto.

I claim:
 1. In a process for removing H₂S from a gaseous stream by thesteps of: (a) contacting the H₂S-containing gaseous stream with asorbing liquor comprising a nonaqueous solvent containing dissolvedsulfur, and a base consisting essentially of a tertiary amine havingsufficient strength and concentration to drive the reaction convertingH₂S sorbed by said liquor and reacting with said dissolved sulfur, toform nonvolatile polysulfide which is soluble in the sorbing liquor; (b)converting the dissolved nonvolatile polysulfide in said sorbing liquorto sulfur which remains dissolved in said liquor by contacting saidliquor from step (a) with an oxidizing gas; (c) converting at least partof said dissolved sulfur in the liquor from step (b) to solidparticulate sulfur; and (d) separating said solid sulfur from step (c)from the liquor; the improvement enabling removal from said liquor ofundesired byproduct thiosulfate and sulfate salt species generated byoxidation of portions of said H₂S; said improvement comprising: addingammonia to said liquor at a point in the said process which issubsequent to said contacting of the liquor with said oxidizing gas, toprecipitate the said undesired species as ammonium sulfate and ammoniumthiosulfate; and separating the solids of the precipitates from saidliquor.
 2. A method in accordance with claim 1 wherein said ammonia isadded by bubbling gaseous ammonia into said liquor.
 3. A method inaccordance with claim 1, wherein said ammonia is added to said liquorbetween steps (b) and (c).
 4. A method in accordance with claim 1,wherein said ammonia is added in sufficient quantities to bring theconcentrations of said dissolved sulfate and thiosulfate salts in saidliquor below a predetermined point.
 5. A process in accordance withclaim 4, wherein the concentration for said salts corresponds to adesired minimum level for free amine in said liquor.
 6. A method inaccordance with claim 1, further including periodically measuring theconcentration of free amine in said liquor and continuing to precipitateand remove said undesired species until the free amine rises to a levelsuch that the steady state flow rate of free amine in said liquor willbe in a ratio to the entering H₂S of from 2:1 to 8:1.
 7. A method inaccordance with claim 1, further including monitoring the electricalconductivity of the liquor from which the said solids of saidprecipitates have been separated, and continuing to precipitate andremove said undesired species until said conductivity falls below apredetermined value.
 8. A method in accordance with claim 1, wherein thesulfate and thiosulfate species are reduced to concentrations of lessthan 0.05M.
 9. A process in accordance with claim 1 further includingrecycling the sorbing liquor separated from the sulfur for furthercontact with the H₂S-containing gaseous stream.
 10. A process inaccordance with claim 9, wherein said nonaqueous solvent has asolubility for sulfur in the range of about 0.05 to 3.0 g-moles ofsulfur per liter of solution.
 11. A process in accordance with claim 10,wherein enough dissolved sulfur remains in said sorbing liquor followingseparation of the precipitated sulfur that when said solution isreturned to the absorber for recycling in the process, a sufficientamount of sulfur is present to react with the inlet H₂S gas.